There are many types of new distributed generation (DG) and energy storage products being developed throughout the world. These include: fuel cells, flywheels, advanced batteries, micro-turbines, Stirling engines, wind turbines, solar cells and double layer capacitors. Each one of these devices requires a power electronic inverter at its output to make useful AC power. Typically, this is 50 or 60 Hz single or three-phase power.
A number of techniques have been described in patents and literature for connecting these devices to each other and to a utility grid. All of these are techniques involve the use of parallel power converters. These converters fall into two categories, devices paralleled on the DC side of the converter or devices paralleled on the AC side of the converter.
The concept of paralleling devices on the DC side permits the use of one large inverter, thereby reducing inverter costs. This motivation for paralleling devices on the DC side is less significant today than in the past, since the cost of controls for multiple inverter systems has decreased significantly. For a larger system, the DC side technique uses a DC distribution system with each distributed generator supplying DC power to the DC distribution system and each load having its own inverter. In this system, a single inverter failure will cause loss of load.
Paralleling devices on the AC side is inherently more reliable, since the loads are AC. No single device failure need drop the AC power to loads as long as there is some excess capacity.
The typical method used to connect a number of power electronics units in parallel is to make one master and the rest slaves. The master is a voltage source and the slaves are current sources. This method works well if the loads are linear, have no quick surges, and draw only real power. When all of these characteristics are not present, problems can arise. These problems can be overcome to some extent through the use of high bandwidth control systems between the paralleled inverters. However, these control systems are not generally applicable for large or disperse systems. In addition, the high speed communication needed between inverters in parallel causes a single point failure issue for parallel redundant power systems and thus makes the master/slave method less reliable.
Equipment has been developed for load sharing between parallel inverters in AC power systems without the use of control circuitry connected to the inverters. Examples of such systems are described in U.S. Pat. No. 5,745,356 to Tassitino, Jr. et al. and U.S. Pat. No. 6,118,680 to Wallace et al. The information needed for load sharing is obtained from the output of each inverter in these systems. The output of each inverter is adjusted based on this information so that all of the inverters in the system equally share the load. Unfortunately, these systems are not believed to share current harmonics and transients, nor do these apparently share reactive current.
One aspect of the present invention is a control system for a controlled current source that receives DC power as an energy input and provides an AC power output for delivery to an AC power network, the AC power output having an output voltage. The control system comprises a voltage signal device, connectable to the controlled current source, for providing a voltage feedback signal representing voltage in the AC power output as an input to control the controlled current source. The control system additional comprises an impedance current regulator for generating an impedance current signal as a function of characteristics of the AC power output from the controlled current source.
Another aspect of the present invention is a system for providing AC power to an AC power network, the system intended for use with a first source that provides a reference AC voltage signal, a second source that provides a real current command signal and a third source that provides a reactive current command signal. The system includes a controlled current source having a source of DC power and a converter for converting the DC power to AC power having an output voltage. The system also includes a current command unit for generating a resultant current command signal. The current command unit is connected to the controlled current source and is connectable to the first and second sources. The current command unit includes an impedance current regulator that provides an impedance current signal and a summing unit for adding the impedance current to the real current command signal from the second source and the reactive current command signal from the third source so as to create the resultant current command signal.
Yet another aspect of the present invention is a method of controlling the operation of a power converter connected to an AC power network that provides an AC power output. The method includes as one step providing a reference AC voltage signal representing output voltage from the power converter. Then, an impedance current command signal is generated, wherein the impedance current command signal is generated based on the reference AC voltage signal. Next, a voltage command signal is generated for controlling the operation of the power converter based on the impedance current command signal and the voltage command signal is provided to the power converter.
Still another aspect of the present invention is a distributed generation network. The network comprises an AC power network for providing AC power and a DC power source for providing DC power. In addition, the network includes a power converter for converting the DC power into AC power. The power converter is connected to the AC power network and the DC power source. The network further includes a control system connected to the power converter for providing a voltage command signal that controls the operation of the power converter. The control system generates (i) a voltage feedback signal representing voltage in said AC power provided by said DC power source and (ii) an impedance current signal as a function of AC power provided by said DC power source. In addition, the control system generates the voltage command signal based on the voltage feedback signal and the impedance current signal.